Among cutting oil agents, used extensively for cutting and grinding, there are mineral-oil-based cutting agents, and water-dilutable O/W emulsion cutting agents, containing e.g. a mineral oil, surfactants and organic amines, and which are used on dilution in water. These cutting oil agents may be added by a compound called an extreme pressure additive for improving the lubricating performance of the oil agents (see for example the Patent Publication 1).
As typical of these extreme pressure additives are chlorine containing compounds, sulfur containing compounds and molybdenum containing compounds. Since waste oils are produced after use of the oil agents for metal machining, chlorine gases, hydrogen chloride gases or sulfur oxide gases may be generated, in case of incineration of the waste oils, thus possibly damaging or shortening the useful life of the incinerating furnaces. Moreover, certain chlorine based additives are known to yield dioxin. On the other hand, strict management is needed for molybdenum compounds which have become the subject of regulations by the PRTR law.
These extreme pressure additives are known as indispensable for improving the lubricating performance of the oil agent. Recently, from the perspective of saving global resources and preventing the worsening of the global environment, it has become a desideratum to develop a lubricant (or coolant) more amenable to the global environment and which will help extend the useful life of e.g. a tool as much as possible.
For overcoming this problem, the present inventors have already devised a high performance lubricating oil comprising an effective amount of fine particles of boron nitride in a liquid component (see for example the Patent Publication 2). These lubricant oils have been confirmed to exhibit superior performance for cutting, grinding and/or polishing, thanks to the meritorious effect of boron nitride content thereof. However, when used as a non-water-soluble cutting agent, these lubricating oils are susceptible to inflammation in spite of superior performance. Hence, in large-sized equipment in need of a large quantity of the oil agent, the general preference is to use water-soluble cutting oils for combating the risk of conflagration. It should be noted that boron nitride, if used as an extreme pressure agent for cutting oils, gives many advantageous features in cost or in operating efficiency, such as prolonged useful life of a blade or improved cutting speed. However, boron nitride suffers from the drawback that it is solid and hence cannot be dissolved in the cutting oil agent, and that, since it has a large specific gravity which is approximately 2.27, it is precipitated if directly dispersed in the cutting oil agent. On the other hand, report has been made of a water dispersion lubricant for plastic machining containing an inorganic solid lubricant (see, for example, the Patent Publication 3). However, this type of the lubricant is limited to use in forging, rolling, line drawing and in extrusion. Additionally, the size of the oil droplet is intended to be fairly large, being 50 μm or more, or 5 μm or more at the minimum, if estimation is made on the basis of the particle size of the solid lubricant used. For metal machining, such as metal cutting, grinding or polishing, a smaller particle size is preferred in light of the machining performance. The particle size of the oil droplet of the routine water-soluble oil agent for metal machining, called the emulsion type, is 2 to 5 μm and is preferably is to be uniformly 5 μm or less in light of use of a filtration device in case of re-use on circulation. This requirement is not met with the water dispersion lubricant. Consequently, there is a demand for a high performance lubricant for metal machining which is water-soluble (or dilutable) and which exhibits superior dispersibility of boron nitride.
[Patent Publication 1]
Publication of JP Patent Kokai JP-A-11-166190
[Patent Publication 2]
JP Patent No. 2911113
[Patent Publication 3]
Patent Publication of JP Patent Kokai JP-A-10-316989
Further, there is strong demand in the art for an efficient process for machining metal, particularly those which are difficult to machine, such as titanium alloys, Inconel or the like, particularly with a less load to the environment.